As to methods for driving and rotating an optical disk by the use of a driver (player), it is known in the art that regarding compact disks (hereinafter, referred to as CDs), a CD is taken out of a case, and after placing it on the turntable of a driver, the CD is mechanically clamped to the turntable by the disk holder, and rotated.
In contrast, regarding optical disks of write-once type (hereinafter, referred to as WO disks) and magneto-optical disks of rewritable type (hereinafter, referred to as MO disks), the disk is inserted into a driver together with its cartridge, and rotated.
More specifically, in the driver, the WO disk or MO disk is drawn and clamped onto the turntable by a magnetic force, which is exerted to a hub attached to the disk, and rotated thereon.
In comparison with CDs, WO disks and MO disks have the advantage that they are protected against scratches and dust since they are kept in the cartridge. Further, WO disks and MO disks have better operability when placed onto and removed from the driver since they are inserted therein together with the cartridge.
Here, the hub attached to the disk has two functions: one for minimizing the eccentricity that occurs between guide grooves formed on the WO disk or MO disk and the rotation center of the turntable; and the other for permitting the WO disk or MO disk to be drawn into position by the magnet installed on the turntable. The guide grooves are provided for allowing the quick-random-access of information on the information area of the WO disk or MO disk. They are formed on the information area in a spiral shape with their center virtually concentric with the rotational axis of the WO disk or the MO disk.
Meanwhile, optical disks of another type, that is, MDs, are shown in a DCC-MD guide book (written by Kinya Murata, Dempa Publications Inc.). Here, DCC refers to Digital Compact Cassette, and MD refers to Mini Disk. As with the WO and MO, the MD is provided with a cartridge for housing an optical disk so as to protect it from scratches and dust.
As illustrated in FIG. 4, in the MD, an optical disk 11, which functions as a recording medium of the magneto-optical method, is housed in a cartridge 12. In the cartridge 12, a hub 13 for clamping the optical disk 11 onto a turntable, not shown, is installed.
Further, a pair of ring-shaped raised portions 14 are formed facing each other inside the cartridge 12. Each raised portion 14 is designed to contact the non-information area of the optical disk 11. Thus, each raised portion 14 protects the information area of the magneto-optical disk 11 from contacting the cartridge case 12, thereby preventing the information area from being damaged.
The hub 13, which is formed to have a plate-shape, is made of a magnetic substance so as to be attracted by the magnet. The only function imparted to the hub 13 is to clamp the optical disk 11 onto the turntable, not shown, by using the magnetic force; therefore, it is not necessary to provide a precise positioning operation such as needed in the WO disk and the MO disk.
Accordingly, as to the positioning operation between the guide grooves and the rotation center of the turntable, it is provided by fitting a center hole 11a to a flattened-cone-shaped projecting portion that is formed on the turntable, which is the same operation as in the CD.
In this arrangement, if the cartridge 12 is vertically placed, or if it is placed upside down, in the case where the hub 13 is not joined to the optical disk 11, the hub 13 tends to come out from the center hole 11a. When this happens, the optical disk 11 might not remain clamped onto the turntable, or the hub 13 might hit against the cartridge 12 upon rotation of the optical disk 11. This results in a drawback wherein the rotation of the optical disk 11 becomes unstable.
In order to eliminate the drawback, there has been suggested an arrangement wherein the hub 13 is fixed along the periphery of the center hole 11a so as to cover the center hole 11a of the optical disk 11. In order to fix the hub 13 along the periphery of the center hole 11a, such methods as using an adhesive or a double-sided adhesive tape have been adopted. Using such methods makes it possible to reduce the manufacturing cost in comparison with the case of the above-mentioned WO disk or MO disk, since no precise positioning mechanism is required. But even such methods result in increased cost since a special fixing device and bonding materials such as adhesives or double-sided adhesive tapes are required.